Ion implantation is a physical process, as opposed to diffusion, which is a chemical process that is employed in semiconductor apparatus fabrication to selectively implant dopant into semiconductor workpieces and/or wafer material. Thus, the act of implanting does not rely on a chemical interaction between a dopant and the semiconductor material. For ion implantation, dopant atoms/molecules are ionized and isolated, sometimes accelerated or decelerated, formed into a beam, and swept across a workpiece or wafer. The dopant ions physically bombard the workpiece, enter the surface and come to rest below the surface and typically come to rest below the workpiece surface in the crystalline lattice structure thereof.
An ion implantation system is a collection of sophisticated subsystems, each performing a specific action on the dopant ions. Dopant elements, in gas or solid form, are positioned inside an ionization chamber and ionized by a suitable ionization process. In one exemplary process, the chamber is maintained at a low pressure (vacuum). A filament is located within the chamber and is heated to the point where electrons are created from the filament source, for example. The negatively charged electrons are attracted to an oppositely charged anode also within the chamber. During the travel from the filament to the anode, the electrons collide with the dopant source elements (e.g., molecules or atoms) and create a host of positively charged ions.
Generally, other positive ions are created in addition to the desired dopant ions. The desired dopant ions are selected from the ions by a process referred to as analyzing, mass analyzing, selection, or ion separation. Selection is accomplished utilizing a mass analyzer that creates a magnetic field through which ions from the ionization chamber travel. The ions leave the ionization chamber at relatively high speeds and are bent into an arc by the magnetic field. The radius of the arc is dictated by the mass and velocity of individual ions, and the strength of the magnetic field. An exit slit of the analyzer permits only one species of ions, the desired dopant ions, to exit the mass analyzer.
Continuing on, the dopant ions are directed towards a target wafer at an end station. The dopant ions, as a beam, impact the wafer with a specific beam current. In order to obtain substantially uniform apparatus characteristics, the beam is required to be substantially uniform and an angle of incidence of the beam is also required to be substantially uniform. It is advantageous to control and/or measure the beam incidence angle accurately, since the electrical characteristics of advanced apparatus are dependent on the beam incidence angle. It is often advantageous to supply beams at incidence angles other than perpendicular to the substrate plane for reasons associated with the geometry or function of the semiconductor apparatus being manufactured.
As apparatus sizes are further reduced, manufacturers require better accuracy in measurements of beam incidence angles in the ion implantation process. Prior art ion beam angular measurement techniques utilize actual angle measurement at the wafer chuck or support hardware. Advanced apparatus require ever increasing precision in the measurement of the angle of the incoming ion beam. Ion beam angle at low energies is difficult to measure using conventional metrology. Typical apparatus utilize a multitude of masks and operations to make a transistor that is sensitive to ion implantation angle.
The angular measurement is typically not made in real time but is done periodically. Angular measurement is typically performed utilizing mechanical tools, laser beams, current measurements, power measurements, etc. However, these techniques have various limitations in terms of accuracy.
For higher energy implantation (e.g., greater that 50 keV) angle can be measured fairly well utilizing the ion channeling effect to measure angle. The lower the energy is below 50 keV the less effective is the resultant angular measurement. However, there is an interest in measuring ion angles for ion beams having energies below 5 keV because it is of practical importance in building advanced semiconductor apparatus and it is the energy range that is typically the most difficult for a commercial ion implanter to control in terms of ion beam angle, for example.
If the ion channeling techniques do not work because of the energy levels involved, manufacturers are forced to build some type of structure that is no longer a bare wafer. That structure has basically been a transistor, such as an NMOS transistor and one or two levels of metal in order to measure beam angle. That involves at least 5 or 6 masks and at least dozen processing steps. Once the structure has been built and implanted it is tested electrically using probes, for example.
Accordingly, suitable apparatus and methods for accurately measuring beam angles in an ion implanter at low energies, at lower cost, etc. are desirable, wherein the apparatus utilize fewer masks and operations to fabricate.